Conventionally, there are many optical communication devices that make use of optical filters using fiber Bragg gratings (FBG) to cut off light over a desired bandwidth (see patent document 1). This FBG utilizes “laser-induced refractive-index change” in which an optical fiber is irradiated with ultraviolet light thereby to increase the refractive index. As the laser-induced refractive-index change is larger, a higher rejection can be obtained. The FBG is a device that creates a periodic perturbation in the refractive index of a fiber core. This periodic perturbation in the refractive index is formed by two-beam interferometry, phase-mask method or the like. With this periodic perturbation in the refractive index, light is reflected in a wavelength range having a center wavelength λB that is called Bragg center wavelength, and is finally rejected in the Bragg center wavelength range. The Bragg center wavelength λB is expressed as λB=2nΛ. Here, n is an effective refractive index of an optical fiber and Λ is a grating pitch which means a period of periodic perturbation in the refractive index. Such FBG is used in a WDM communication system as well as a multiplexer/demultiplexer, a line monitoring filter, a temperature sensor and a distortion sensor.
Patent document 1: Japanese Patent Laid-open Publication No. 2002-328238